Javascript Quiz with Answers

programming review

Table of Contents:

1. Who created JavaScript language?

☑ Brendan Eich ☐ Douglas Crockford ☐ James Gosling ☐ Ryan Dahl

  • Brendan Eich created the very first JavaScript engine for the Netscape Navigator Web browser.
  • Douglas Crockford specified JSON format.
  • James Gosling originally developed the Java programming language.
  • Ryan Dahl wrote Node.js.

2. JavaScripts works inside web browsers only?

☐ YES ☑ NO

No. It works not only inside web browsers.

Today, JS engines are embedded in many other types of host software, including server-side in web servers and databases, and in non-web programs such as word processors and PDF software.

Today you can also write mobile and desktop applications in JavaScript.

3. What is JavaScript?

☐ Language that follows ECMAScript standard
☐ Client-side scripting language (web browsers)
☐ Server-side scripting language
☑ All these

ECMAScript presents a standard or set of rules that JavaScript follows, and tries to adopt over time.

Since JavaScript works inside browsers (Firefox, Chrome, Microsoft Edge), developer teams working on browsers are responsible for adopting ECMAScript standards.

Also JavaScript is the scripting language on the server-side (Node.js).

4. Developers first reference on JavaScript should be MDN (Mozilla Developer Network)

☑ True ☐ False

True. MDN (Mozilla Developer Network) is a resource for developers, maintained by the community of developers and technical writers.

MDN has many documents on a wide variety of subjects, such as: HTML5, JavaScript, CSS, Web APIs and more.

5. Is JavaScript Object Oriented language?

☑ YES ☐ NO

Yes. JavaScript supports Object Oriented Programming (OOP) because it supports inheritance through prototyping.

JS can create polymorphism, encapsulation and many sub-classing paradigms.

6. JavaScript is the scripting language of Java?

☐ True ☑ False

JavaScript should not be confused with the Java programming language.

The two programming languages have similar syntax, but one cannot say JavaScript is the scripting language of Java. They are not that related.

7. JavaScript has primitive (immutable) data types and mutable object types.

☑ YES ☐ NO

Yes. In JavaScript, there are 6 primitive data types: string, number, boolean, null, undefined, and symbol.

Primitive types define immutable values. For example, if we set:

var a = 1;

and later on we set

a = 2;

Inside JavaScript runtime, variable a that is first equal to 1 will be destroyed and new variable a will be created to hold the value of 2.

Objects on the other side are mutable. You can set a property to the object, but it will not be destroyed.

8. Inside what HTML tag you would put JavaScript code?

☐ <js>
☐ <scripting>
☑ <script>
☐ <javascript>

The HTML <script> tag is used to embed or reference JavaScript code.

Tags: <js><scripting><javascript> are fake.

9. Where is the correct place to insert JavaScript on a web page?

☐ Inside <body>
☐ Inside <head>
☑ Inside <head> and <body>
☐ None of these

The correct place to insert JavaScript on the web page is both:

<head> section and <body> section.

10. What is the correct syntax to use an external script called “new.js”?

☑ <script src="new.js">
☐ <script name="new.js">
☐ <script href="new.js">
☐ <script content="new.js">

The correct answer is: <script src="new.js"></script>

If the "src" attribute is present, content inside the <script></script> tags must be empty.

These are fake:

  • <script name="new.js">
  • <script href="new.js">
  • <script content= "new.js">

The name, href, content are not <script> tag attributes.

11. The external JavaScript file must contain the <script></script> tags.

☑ False ☐ True

External file is called like this:

<script src="new.js"></script>

We don’t need to have a wrapper <script></script> in new.js.

Note: External JavaScript files need to have .js extension.

12. What is correct commenting in JavaScript?

☑ /*This is a comment*/
☐ <!--This is a comment-->

In JavaScript you can write single line comments with // or multiline comments with /* */.

For example:

// This is single line comment
 
/*This comment is
multiline*/

Note: This is how you comment in HTML

<!--This is also a comment-->

13. How do you declare a JavaScript variable x?

☐ define x;
☐ variable x;
☑ var x;
☐ def x;

Correct is to use var x;

The var statement declares a variable, optionally initializing it to a value.

Note: In var x; we just declared a variable x. We haven’t initialized it.

To initialize a variable x you would write like this:

var x = "quiz";

14. To get the data type of some variable you will use:

☐ dataType ☑ typeof

The typeof operator is used to get the data type. The operand can be either a literal or a data structure such as a variable, a function, or an object. The operator typeof returns the string.

Following lines are both correct:

  • typeof operand
  • typeof(operand)

15. When we don’t assign a value to a variable it will be?

☐ null
☑ undefined
☐ ""
☐ NaN

A variable that has not been assigned a value holds the value undefined.

Expression null represents the intentional absence of any object value or type.

If we write:

var x = null;

We can say: x is known to exist, but it has no type or value.

16. In JavaScript can we pass functions as arguments to other functions?

☑ YES ☐ NO

Yes. This means JavaScript supports passing functions as arguments to other functions, returning them as the values from other functions, and assigning them to variables or storing them in data structures.

This feature is called first-class functions.

17. Would the next code cause the error?

var y;
y = 9;
y = "Dog";

☐ YES ☑ NO

No. JavaScript supports dynamic types. This means that the same variable can be used to hold different data types.

In our case:

var y;
y = 9;
typeof y; // returns "number"
y = "Dog";
typeof y; // returns "string"`

The variable y will first hold the number data type variable and later the string.

This is why we also say JavaScript is loosely typed.

18. Can you guess the return of the following code:

Boolean(10>9)

☑ true
☐ NaN
☐ false
☐ Undefined

It will return true.

Note: In JavaScript primitive boolean values true and false are not the same as Boolean object values true and false.

The Boolean object is an object representation of a primitive boolean type.

The value passed as the first parameter will be converted to Boolean object true or false.

Boolean(arg) will return true if arg is either one of these:

omitted
0
-0
null
false
NaN
undefined
the empty string ("").

Else, Boolean objects will be initialized as false.

19. How do you write “Eureka” in an alert box?

☐ msg("Eureka");
☑ alert("Eureka");
☐ msgBox("Eureka");
☐ alertBox("Eureka");

The alert() method displays an alert box with a specified message and the OK button.

Note: The alert box takes the focus away from the current window, and forces the browser to read the message.

Do not overuse this method, as it prevents the user from accessing other parts of the page until the box is closed.

20. What is the correct way to write a JavaScript array?

☑ var num=[1,2,3];
☐ var num=(1,2,3);

In JavaScript we create arrays in two ways:

Array literal syntax:

var arr = [item1, item2, ...];

Using the new keyword:

var arr = new Array(item1, item2, ...);

Array items typically have different data types.

21. var x = 3 + 4 + "7"

The value of x is :
☐ 14
☑ "77"
☐ "347"
☐ 77

The correct result is "77".

Inside var x = 3 + 4 + "7" the first + is an addition operation 3+4=7, and the other + is a concatenation 7+"7" that returns the string "77".

22. What is the result of the comparison?

var x = "Ted";
var y = new String("Ted");
x === y

☐ True ☑ False

The operator === returns true if the operands are equal by value and by type.

The variables x and y have different types.

The type of variable x is string. The type of the variable y is object.

23. What will be the variable lastNum?

var num = [1, 2, 3];
var lastNum = num[num.length];

☐ 3 ☑ undefined

The variable lastNum will be undefined.

Note: In JavaScript indexing starts from 0. num.length will return 3, because there are 3 elements.

To get the last element of the array we should write: num[2]; or num[num.length - 1];

24. Which JavaScript event occurs when the user clicks an HTML element?

☑ onclick ☐ onmouseclick

onclick - the event occurs when the user clicks the element. onmouseclick - doesn’t exist

25. What is the intruder?

☐ mouseover
☐ mouseout
☑ mousein
☐ mousemove

The mousein is not a standard event in JavaScript.

Events:

  • mousemove - is when the pointer is moving while it is over an element
  • mouseout - is when a user moves the mouse pointer out of an element, or out of one of its children
  • mouseover - is when the pointer is moved onto an element, or onto one of its children

26. Can you test JavaScript code inside web browsers like Firefox, Chrome, Microsoft Edge?

☑ YES ☐ NO

Yes. All you need is:

  • Press F12 to open the Inspector
  • Select the Console tab
  • Start typing your JavaScript and press Enter when done.

27. What is the return of the next comparison:

null === undefined

☐ True ☑ False

The operator === returns true if the operands are equal by value and by type.

The null and undefined are equal by value but not by type.

typeof(null) // returns "null"
typeof(undefined) // returns "undefined"

28. Is this correct?

var int number = 12;

☐ YES ☑ NO

JavaScript does not define different number types (integers, short, long, floating-point etc). There is only a single “number” type.

Numbers can be written with or without decimals.

JavaScript numbers are always stored as double precision floating point numbers (64 bit representation).

29. Are semicolons required at the end of the JavaScript line?

☐ YES ☑ NO

No, semicolons are not needed at the end of line.

A semicolon at the end of a line indicates where a statement ends. If you need to separate statements on a single line they are required.

Some people believe that it is a good practice to put them at the end of each statement.

30. To add an element to the end of an array you use:

☐ pop()
☐ add()
☑ push()
☐ addToEnd()

  • push() method adds a new element to the end of an array.

  • pop() method removes the last element from an array.

  • add() and addToEnd() are not standard commands in JavaScript.

31. Which of the following events will you add in the addEventListener() method?

☑ click
☐ onclick

The click is an event listener used for the addEventListener() method.

Example:

object.addEventListener("click", myFunction);

The onclick is an event handler.

The syntax for onclick is like this:

object.onclick = function(){ ... };

32. A string can be converted to an array using which method:

☑ split()
☐ slice()
☐ splice()
☐ piece()

The method to convert string to an array is called split().

Here is the example:

var text= "a,b,c,d,e";
var arrText = text.split(",");
// The variable arrText is an array object.
// arrText=["a","b","c","d","e"];

33. Does the addEventListener() method allow you to add many events to the same element?

☑ YES ☐ NO

Yes, the addEventListener() method allows you to add many events to the same element, without overwriting existing events.

x.addEventListener("mouseover", func1);
x.addEventListener("click", func2);
x.addEventListener("mouseout", func3);`

34. Can we convert the string

"5 dogs" to number with the method: parseInt("5 dogs");?

☑ YES ☐ NO

Yes, in this case it is possible.

The function parseInt() parses a string and if it is possible converts to number, otherwise returns NaN (Not a Number).

For instance:

parseInt("dogs 5"); // will return NaN

35. What is the output of this code?

var letters = ['a', 'b'];
letters.slice(1);        
console.log(letters);

☐ Array['b']
☑ Array['a','b']

The statement letters.slice(1) slices out a part of the array letters starting from array element 1.

console.log(letters.slice(1));
//returns Array['b']

The slice() method creates a new array. It will not remove any elements from the source array.

console.log(letters);
//returns Array ['a','b']

36. Can we declare a variable if we have used it before?

☑ YES ☐ NO

A variable can be declared after it has been used.

a = 3;
var a;

Note: Variables declared without the var keyword are global. Variables that are defined using the var keyword will be local to the scope. If they have been declared inside the function they will be local for that function.

Generally, the coding style above is not a good practice. We should always tend to declare variables early and initialize them early.

37. Moving declarations to the top of the script is called hoisting

☑ YES ☐ NO

Yes. Hoisting is JavaScript’s default behavior of moving all declarations to the top of the current scope (to the top of the current script or the current function).

38. If a function is called with a missing argument, the value of the missing argument is set to undefined.

☑ YES ☐ NO

Yes. Sometimes it is a good habit to assign default values to arguments. For example:

function sum(a, b){
    return a + b;
}

If we call sum(0,) we will get NaN.

0+undefined will evaluate to NaN.

39. What is the result of the comparison using the == operator ?

var a = 5;
var b = "5";
a==b

☑ true ☐ false

In regular comparison ==, data type does not matter.

Variables a and b are equal by value and the result of this comparison is true.

40. What is the result of the comparison using the === operator ?

var a = 5;
var b = "5";
a===b

☐ True ☑ False

In === comparison, data type does matter.

Variables a and b are not equal by type and the result of this comparison is false.

41. Variables created without the keyword var, are always global, even if they are created inside a function.

☑ YES ☐ NO

To make sure that the variable belongs to the desired scope, always use the keyword var.

function x() {
a = 5;
var b = 10;
}

Since it is defined without the keyword var the variable a is global although it is in the function.

42. Can we define the function as follows?

var x = function(a, b){
   return a*b;
}

☑ YES ☐ NO

This is an example of an anonymous function where the function name is omitted.

Note: Variable x holds the function object.

Recall that in JavaScript you can put a function into a variable and pass that variable to another function.

43. Which of the following properties return the URL of the current page.

☐ location.URL
☐ URL.location
☑ window.location.href
☐ window.location.hostname

The window.location.href property returns the URL of the current page.

The window.location.hostname property returns the domain name of the web host.

The location.URL and the URL.location are fake.

44. How can you detect the client’s browser type?

☐ navigator.platform
☐ navigator.appName
☑ navigator.userAgent
☐ browser.appName

The navigator object contains information about the browser.

The navigator.userAgent property returns the type of the browser.

Here is how it looks for Mozilla Firefox version 60:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0

The navigator.appName property returns the traditional name of the browser application. Typical appNames:

Opera, Netscape, Microsoft Internet Explorer

The navigator.platform property returns the browser platform. Typical platforms are:

HP-UX
Linux i686
Linux armv7l
Mac68K
MacPPC
MacIntel
SunOS
Win16
Win32
WinCE

The browser.appName is fake.

45. You need to check the Internet connectivity status in a browser. Which method should you use?

☐ navigator.isOnline ☑ navigator.online

The navigator object contains information about the browser.

The navigator.online property detects if the browser is online. It returns boolean true if the browser is online, otherwise false.

46. Find the intruder:

☐ Function()
☐ Number()
☑ Math()
☐ Boolean()

Math() is not a constructor. The new keyword cannot be used in Math class.

All properties and methods of Math are static.

Function(), Number() and Boolean() are built-in constructors.

47. The method confirm('message') will show:

☑ dialog box ☐ input field

The confirm() method displays a dialog box with a specified message and buttons OK and a Cancel.

48. Which timing event is used to delay the execution of the function?

☐ timer() ☑ setTimeout()

We use setTimeout() to create a delay.

Here is how we call the function:

setTimeout(function, milliseconds);

The first parameter is a function we plan to execute.

The second parameter indicates the number of milliseconds before execution.

49. What would be the result?

document.getElementsByTagName("p");

☐ array
☑ HTMLcollection
☐ number
☐ paragraph

The getElementsByTagName() method returns an HTMLCollection object.

This collection will represent all paragraphs(<p> tags) on the web page.

50. Choose a way to add a new property “price” to the constructor “Shoes”.

function Shoes(color, size) {
   this.color = color;      
   this.size = size;        
}

☑ Shoes.prototype.price=100; ☐ Shoes.price=100;

All JavaScript objects inherit properties and methods from a prototype.

The JavaScript prototype property allows you to add new properties to object constructors.

function Shoes(color, size) {
    this.color = color;
    this.size = size;
}
 
Shoes.prototype.price = 100;
var myShoes = new Shoes("blue", 10);
Console.log(myShoes.color); //returns "blue"
Console.log(myShoes.price); //returns 100

…

tags: quiz - answers & category: javascript